The present invention relates generally to articulated tractors and, more particularly, to a control linkage for effecting a shifting of a transmission positioned proximate to the articulation axis of the tractor.
Articulated tractors typically include two halves joined to each other by pivot mechanism defining a generally vertically oriented articulation axis, with each half being supported by a pair of ground engaging wheels rotatably mounted in a fixed orientation relative to each tractor half to provide a mobile support of the articulated tractor over the ground. Steering is accomplished by pivoting the two halves relative to each other about the articulation axis, usually by actuation of one or more hydraulic cylinders interconnecting the two tractor halves on opposing sides of the articulation axis, as compared to the steering action of a non-articulated tractor which is accomplished through a pivotal movement of at least one pair of wheels.
In typical configurations, one-half of the articulated tractor carries the power components, such as the engine, transmission, and major drive assemblies, while the other half of the articulated tractor supports the operator's station, including control levers, steering wheel, switches, etc. for controlling the operation of the tractor and operative components thereof. The transmission on such articulated tractors provides two or more speed ranges for motive operation of the tractor at any given output from the engine. Some such tractors are hydraulically driven in that the engine drives a hydraulic pump which in turn delivers a flow of hydraulic fluid under pressure to a hydraulic motor operably connected to the transmission.
The transmission will have a pivotable shift control arm which is manipulated to change the speed ranges of the transmission and which is connected to a range select lever in the operator's control station by a push/pull cable. An inherent problem with cable controls on an articulated vehicle in which the operator's control station is supported on one-half of the tractor while the transmission is located on the other half of the tractor on the opposite side of the articulation axis is that the articulating nature of the tractor applies many cycles of reversed bending loads on the push/pull cable itself. This type of loading has a dramatic effect on the fatigue life of push/pull cables. This problem is heightened by spatial configurations in which the transmission is supported on the frame of the articulated tractor adjacent the articulation axis.
One known solution to improving the fatigue life of cables in environments as described above has been to use an extra long cable that loops around the transmission housing and back to connect to the shift control arm of the transmission from the opposite direction. However, because of the high friction losses in such an extra long cable making several bends to loop around the transmission housing, the shifting of the transmission between ranges was made more difficult and created a "dead feel" because of the high friction loss. Furthermore, the requirement of an extra long cable adds undesirable additional costs to the manufacture of such a transmission control linkage.
Accordingly, it would be desirable to provide a transmission control linkage for articulated tractors, in which the transmission housing is located proximate to the articulation axis, which will eliminate the aforementioned fatigue problem while maintaining proper shift feel and operation.